Primary accretion of large planetesimals from chondrule size particles
Seminar Room 1, Newton Institute
Primary accretion is the process by which the first large objects formed from freely floating nebula particles. Several clues as to the nature of this process are to be found in primitive meteorites and asteroids. The most primitive chondritic meteorites display a characteristic texture: predominance of mm-sized, once-molten chondrules, metal grains, and refractory oxide particles, each surrounded by fine-grained dust rims and all embedded in a granular matrix. The size distribution of the chondrules in all classes of chondrite is quite narrow and nearly universal in shape, but with a mean size distinctive of each class.
At least two entire chondrite classes are each thought to derive from only one or two planetesimals, roughly 100 km in size and originally composed largely of chondrules with very similar properties. This ubiquitous and unusual texture is surely telling us something important about primary accretion, but there is no explanation for it at present.
Moreover, the extended duration of meteorite parent body formation as revealed in isotopic age-dating, and the scarcity of melted asteroids, suggest that primary accretion went on for a long time. We have shown how well-sorted, chondrule-sized mineral particles can be concentrated, by orders of magnitude, into dense zones in weak nebula turbulence. This turbulent concentration explains the characteristic size and size distribution of chondrules in a natural way.
We developed a cascade model of the statistics of dense zones and their correlation with gas vorticity, which incorporates the effects of particle mass loading on the gas and predicts the fractional volume of particle-rich zones which can evolve directly into objects with some physical cohesiveness. We have derived threshold conditions (combinations of particle density, clump lengthscale, gas density, and local vorticity) which allow dense clumps to proceed to become actual planetesimals. Combination of these thresholds with our cascade models recently led us to a method for predicting the relative abundance of primary planetesimals as a function of mass - their birth function - and even their production rate. The predictions can be extended easily from the asteroid belt to the Kuiper belt; similar size populations are found to arise.
A number of challenges remain in validating and solidifying this scenario. The key elements of the cascade model must be validated (or modified) using deeper inertial ranges, further from the dissipation scale. The settling of dense clumps in the vertical component of solar gravity increases the local density of chondrule-size components in regions near the midplane, and must be modeled. Finally, the self-gravity of dense particle clumps in turbulence must be modeled to assess their stability and mutual interactions.